The present disclosure relates to a composition with which a coloring material for presenting a structural color and colloidal crystals usable as an optical element can be produced, a colloidal crystal film formed of the composition, and a method for manufacturing the colloidal crystal cured film. A composition for colloidal crystals and a colloidal crystal cured film according to the present disclosure are useful for applications in the optical technical field, such as optical elements and optical functional materials.
An accumulation of monodisperse particles three-dimensionally regularly arranged is referred to as colloidal crystals. The colloidal crystals cause the diffraction or interference of incident light and, depending principally on its periodic structure, reflect light having a particular wavelength (Bragg reflection). For example, colloidal crystals of submicron particles reflect ultraviolet light to visible light and infrared light, depending on the particle size. When the reflected light has a wavelength in the visible light region, the color of the colloidal crystals can be visually recognized as a structural color. Colloidal crystals have been actively studied and are expected to be used in various optical elements and optical functional materials, such as photonic crystals. For example, colloidal crystals can be applied to various coloring materials, such as paints, inks, and cosmetics, optical filters, optical memory materials, display devices, optical switches, sensors, and lasers.
Many studies have reported methods for producing colloidal crystals. Colloidal crystals are roughly divided into “closest packed (hard)” and “non-closest packed” colloidal crystals. “Non-closest packed” colloidal crystals are further divided into “non-closest packed (soft)” colloidal crystals and “non-closest packed (semi-soft)” colloidal crystals.
“Closest packed (hard)” colloidal crystals are an accumulation of particles, for example, of silica or polystyrene closely packed, and particles in the colloidal crystals are in contact with one another. Crystals can be grown through the accumulation of particles associated with the evaporation of water-soluble solvent, yielding dry colloidal crystals. Thus, an infinite number of voids are present between the particles. The hard colloidal crystals are composed of particles accumulated only by contact and therefore have very low mechanical strength, and are broken by the action of a slight external force. Thus, according to one disclosed production process, in order to fill the voids between particles with a binder, such as a monomer or a polymer, the binder is applied to the colloidal crystals (see, for example, JP2005-60654A). According to another disclosed production process, core-shell particles prepared by two-stage emulsion polymerization in which the surface of the core (particle) is coated with a resin of the shell are used to fill the voids between particles with the shell (see, for example, JP2009-249527A).
According to still another disclosure, “non-closest packed (soft)” colloidal crystals can be produced by removing an ionic substance from a particle dispersion containing a water-soluble solvent as a dispersion medium (see, for example, JP06-100432A). According to this production process, deionization expands an electric double layer on the particle surface and causes electrostatic repulsion between the particles. This suppresses Brownian movement of the particles and regularly arranges the particles throughout the dispersion medium. Since “soft” colloidal crystals contain a liquid dispersion medium, the regular arrangement of particles is easily broken by a slight external force, such as vibrations, or a temperature change. Thus, in order to put “soft” colloidal crystals to practical use as a material, the colloidal crystals must be immobilized while the regular arrangement of particles is maintained. According to another disclosed production process, a small amount of water-soluble monomer is added to a water-soluble solvent, and the monomer is polymerized to immobilize colloidal crystals with the polymer gel (see, for example, JP2007-29775A). An immobilization method using no solvent is also disclosed (see JP2008-303261A).
“Non-closest packed (semi-soft)” colloidal crystals contain core-shell particles in which a linear polymer of the shell is bonded to the surface of the particle (core). The core-shell particles prepared by two-stage emulsion polymerization described in JP2009-249527A, in which the surface of the core is coated with a resin of the shell, are different from the core-shell particles used in “semi-soft” colloidal crystals. The linear polymer of the core-shell particles used in “semi-soft” colloidal crystals can dissolve in an organic solvent. However, the linear polymer bonded to the surface of the core is not detached from the core and contributes to the dispersion stabilization of the core. In the dispersion of core-shell particles in an organic solvent, the steric repulsion of a linear polymer or osmotic effects prevent the aggregation of particles, thus forming colloidal crystals (see, for example, WO 2005-108451). Restrictions on the solvent usable as a dispersion medium are less stringent in the case of “semi-soft” colloidal crystals than in the case of “soft” colloidal crystals. Thus, “semi-soft” colloidal crystals can form colloidal crystals even in a hydrophobic organic solvent or a hydrophobic monomer. “Semi-soft” colloidal crystals are also superior to “soft” colloidal crystals in that the particle spacing in a crystalline state can be controlled via the molecular weight of the linear polymer. However, “semi-soft” colloidal crystals contain a liquid dispersion medium as in “soft” colloidal crystals. Thus, in order to put “semi-soft” colloidal crystals to practical use, the colloidal crystals must be immobilized while the regular arrangement of particles is maintained. According to another disclosed method, a small amount of monomer is added to the organic solvent of “semi-soft” colloidal crystals and is polymerized to immobilize the colloidal crystals with the polymer gel (see, for example, WO 2003-100139).
In a drying process of a particle dispersion for “hard” colloidal crystals described in JP2005-60654A, the addition of a hydrophobic binder causes the aggregation of the particles, resulting in an irregular arrangement of the particles (JP2009-249527A (see p. 14)). Although a binder is applied to colloidal crystals thus formed, the colloidal crystals may be broken. In order to avoid this, according to one disclosed production process, an adhesive layer is formed on a substrate in advance. However, this requires complicated procedures with many steps. Furthermore, it is difficult to completely fill the voids between particles with the binder, and disordered voids often remain. The residual disordered voids cause light scattering and consequently cloudiness in the resulting colloidal crystals. The hard colloidal crystals tend to have an infinite number of cracks in its coating film during the drying process. This is because, with decreasing interparticle distance during the drying process, uneven shrinkage results in the breakage of part of the crystals, thus causing cracks. These cracks have various sizes and may be large enough for visual observation or may be microcracks of several micrometers, which are difficult to visually observe. An infinite number of these cracks also scatter light and cause cloudiness in the resulting colloidal crystals. According to one disclosed method, in order to prevent cracking, colloidal crystals are formed on a substrate having trench isolation. This method requires a substrate having a particular shape (see, for example, JP2005-60654A). According to the method using the core-shell particles synthesized by the two-stage emulsion polymerization described in JP2009-249527A, the resin that fills the voids is a polymer having less flowability than low-molecular binders. It is therefore difficult to completely fill the voids with the resin. Furthermore, it is difficult to prevent all cracks only with the polymer of the shell. Thus, there are problems that the immobilization of colloidal crystals requires complicated procedures or substrates, and residual voids or cracks cause cloudiness.
“Soft” colloidal crystals described in JP2007-29775A and JP2008-303261A are formed utilizing the electrostatic repulsion of the surface charges of particles. This requires the use of a water-soluble solvent having a high dielectric constant as a dispersion medium. Thus, the colloidal crystals are immobilized only in a gel state in the presence of the water-soluble solvent. The “semi-soft” colloidal crystals including the core-shell particles described in WO 2005-108451 and WO 2003-100139 are formed using a hydrophobic monomer as a dispersion medium. Thus, the colloidal crystals are supposed to be immobilized as a cured film by curing (polymerizing) the monomer. However, a regular arrangement of particles is likely to be broken during the polymerization of the monomer. Thus, there is a problem that the optical properties cannot sufficiently be maintained through the curing. This is probably because of the mass transfer or cure shrinkage of the monomer during the polymerization. Thus, the regular arrangement of particles is difficult to maintain during the immobilization of the colloidal crystals. The colloidal crystals immobilized by the method described in JP2010-18760A are immobilized as a gel containing an organic solvent, which constitutes approximately 50% of the gel. Thus, colloidal crystals immobilized as a gel containing a water-soluble solvent or an organic solvent have low mechanical strength and are therefore difficult to put to practical use in optical functional materials. Furthermore, the evaporation of the solvent may disorder the regular arrangement of particles or change the interparticle distance. This changes the reflection wavelength, resulting in poor stability. According to the method described in JP2008-303261A, the solvent content is 30% by weight or less, and a monomer used as a binder is limited to a water-soluble poly(alkylene glycol) monomer.
In order to solve such problems, the present inventors previously found a method for manufacturing a cured film for presenting a bright structural color by limiting the solubility parameters of a shell of core-shell particles (A) and a binder monomer (B) in “semi-soft” colloidal crystals and limiting the acrylic equivalent of the monomer (B) JP2010-18760A. Advantageously, a cured film manufactured by this method has high mechanical strength and excellent stability because of the absence of solvent evaporation and can form a film for presenting a structural color by a simple method.
However, the colloidal crystal cured film disclosed in JP2010-18760A suffers from still insufficient curing and has a high haze because of slight residual cloudiness.
When a colloidal crystal cured film for presenting a structural color is used as a coloring material, such as a paint, necessary optical properties include sufficiently high reflectance at a reflection peak presenting the color, a low haze, and high transparency. The reflectance at a reflection peak may be increased by increasing the film thickness and the number of colloidal crystal layers. However, this also increases the haze. In general, in order to use a colloidal crystal cured film as a coloring material, the reflectance at a reflection peak must be 50% or more, and the haze must be 10% or less.
Thus, there has been a need for improved compositions for colloidal crystals.